In the fabrication of modern integrated circuit devices, one of the key requirements is the ability to construct plugs or interconnects in reduced dimensions such that they may be used in a multi-level metalization structure. The numerous processing steps involved require the formation of via holes for the plug or interconnect in a dimension of 0.5 .mu.m or less for use in high-density logic devices. For instance, in forming tungsten plugs by a chemical vapor deposition method, via holes in such small dimensions must be formed by etching through layers of oxide and spin-on-glass materials at a high etch rate. A high-density plasma etching process utilizing a fluorine chemistry is frequently used for the via formation process.
The via hole formation process can be enhanced by improving the etch directionality by a mechanism known as sidewall passivation to improve the anisotropy of the etching process. By utilizing a suitable etchant gas and reactor parameters, an etch-inhibiting film, normally of a polymeric nature, can be formed on vertical sidewalls. The etch-inhibiting film or the polymeric film slows down or completely stops any possible lateral etching of horizontal surfaces in the via hole. For instance, when a fluorine-containing etchant gas such as CFH.sub.3 is used, a fluorine-type polymeric film is formed on the sidewalls. Many photoresist materials may also contribute to the formation of a polymeric film on the sidewalls. After the sidewall is coated with a polymeric film, it is protected by the inhibitor film to preserve the line width or via hole diameter control.
In a modern etch chamber, an electrostatic wafer holding device, i.e., an electrostatic chuck or E-chuck, is frequently used in which the chuck electrostatically attracts and holds a wafer that is positioned on top. The E-chuck holding method is highly desirable in the vacuum handling and processing of wafers. In contrast to a conventional method of holding wafers by mechanical clamping means where only slow movement is allowed during wafer handling, an E-chuck device can hold and move wafers with a force equivalent to several tens of Torr pressure.
In an etch chamber equipped with a plasma generating device and an electrostatic chuck for holding a wafer, a shadow ring is utilized as a seal around the peripheral edge of the wafer. The shadow ring, sometimes known as a focus ring, is utilized for achieving a more uniform plasma distribution over the entire surface of the wafer and to help restrict the distribution of the plasma cloud to stay only on the wafer surface area. The uniform distribution function is further enhanced by a RF bias voltage applied on the wafer during a plasma etching process. Another function served by the shadow ring is sealing at the wafer level the upper compartment of the etch chamber which contains the plasma from the lower compartment of the etch chamber which contains various mechanical components for controlling the E-chuck. This is an important function since it prevents the plasma from attacking the hardware components contained in the lower compartment of the etch chamber. In order to survive the high temperature and the hostile environment, the shadow ring is frequently constructed of a ceramic material such as quartz.
A typical inductively coupled plasma etch chamber 10 is shown in FIG. 1. In the etch chamber 10, which is similar to a Lam TCP etcher, the plasma source is a transformer-coupled plasma source which generates a high-density, low-pressure plasma 12 decoupled from the wafer 14. The plasma source allows an independent control of ion flux and ion energy. Plasma 12 is generated by a flat spiral coil 16, i.e., an inductive coil separated from the plasma by a dielectric plate 18, or a dielectric window on top of the reactor chamber 20. The wafer 14 is positioned sufficiently away from the coil 16 so that it is not affected by the electromagnetic field generated by the coil 16. There is very little plasma density loss since plasma 12 is generated only a few mean free paths away from the wafer surface. The Lam TCP plasma etcher enables a high-density plasma and high etch rates to be achieved. In the plasma etcher 10, an inductive supply 22 and a bias supply 24 are used to generate the necessary plasma field. Multi-pole magnets 26 are used for surrounding the plasma 12 generated. A wafer chuck 28 is used to hold the wafer 14 during the etching process. A ground 30 is provided to one end of the inductive coil 16.
In a typical inductively coupled RF plasma etcher 10 shown in FIG. 1, a source frequency of 13.56 MHZ and a substrate bias frequency of 13.56 MHZ are utilized. An ion density of approximately 0.5.about.2.0.times.10.sup.12 cm.sup.3 (at wafer), an electron temperature of 3.5.about.6.0 eV and a chamber pressure of 1.about.25 m Torr are achieved or used.
In the plasma etcher shown in FIG. 1, when the chamber is idled for any extended length of time, i.e., more than half hour, the etch rate in a semiconductor material layer may drop when production is resumed in the chamber. For instance, such drop has been observed in a doped Poly etching process which leads to incomplete etching of a doped poly layer and subsequently, doped poly residue on the surface of a wafer. This type of defect is frequently known in a fab plant as a "first wafer event". The wafers are frequently scrapped due to the defect. A normal practice in the fab plant is to use season wafers in etch chambers during a pre-heating step after the chamber has been idled more than half hour. Even with the use of season wafers, the doped poly residue problem still exists. It is assumed that the "first wafer event" defect is caused by a decrease in the etch rate of the doped poly since the regular recipe is no longer adequate for etching away the doped poly layer. When an oxide layer is subsequently deposited on top of the poly layer, defects occur on the surface of the wafer. Empirically, it has been observed that after an etch chamber has been idled for about half hour, the temperature of the top plate which includes the inductive coil and the dielectric window has dropped to a temperature of approximately 33.about.34.degree. C. from its operating temperature of 43.about.44.degree. C. The longer the idle time of the process chamber, the worse is the yield of the wafer.
It was also observed that after a few wafers have been etched in the plasma etch chamber, the inductive coil generates enough heat to heat up the dielectric window, i.e., the quartz disc, to a temperature of approximately 43.about.44.degree. C. which is suitable for the doped poly etching process. A suitable process condition is therefore achieved for the etch chamber to function properly even though the first few production or season wafers are wasted. A cause for the rapid cooling of the etch chamber during idle is that an exhaust from the chamber continues through the idle period which takes heat away from the chamber components.
It is therefore an object of the present invention to provide an apparatus for preventing etch rate drop after machine idle in a plasma etch chamber that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for preventing etch rate drop after machine idle in a plasma etch chamber by utilizing a heating means for heating a top plate in the etch chamber that includes a conductive coil and a dielectric window.
It is a further object of the present invention to provide an apparatus for preventing etch rate drop after machine idle in a plasma etch chamber by providing an enclosure for enclosing a top plate in the plasma chamber and a heating means for heating air in the enclosure such that the top plate in the etch chamber is maintained at a temperature of approximately 40.degree. C.
It is another further object of the present invention to provide an apparatus for preventing etch rate drop after machine idle in a plasma etch chamber which includes an enclosure for enclosing a top plate in the etch chamber, a heater housing, at least one heat lamp positioned in the heater housing, and a blower for delivering heated air into the enclosure cavity.
It is yet another object of the present invention to provide a plasma etch chamber that is equipped with an apparatus for preventing etch rate drop after machine idle that includes an etch chamber and an enclosure positioned on top of the etch chamber for enclosing and heating a top plate of the chamber and maintaining the etch rate carried out in the chamber.
It is still another further object of the present invention to provide a method for preventing etch rate drop after machine idle time in a plasma etch chamber by providing an etch chamber equipped with an enclosure for enclosing a top plate of the chamber and for heating the top plate by a heating device.
It is yet another further object of the present invention to provide a method for preventing etch rate drop after machine idle time in a plasma etch chamber by providing a heating means for a top plate of the chamber capable of generating a heated air at a temperature sufficient to maintain the top plate at between about 35.degree. C. and about 45.degree. C. when the heated air is flown to the top plate.